In today's data communication, an amount of data is divided into individual units, which units are transmitted to a desired receiver over an appropriate communication path.
This form of data communication is very well known and in wide use. The sending node may e.g. be a radio base station and the receiving node may be a user equipment such as a mobile phone, portable computer, Personal Digital Assistant (PDA) or vice versa. Most of these systems use bi-directional radio communication where both nodes transmit and receive data units simultaneously or alternating.
Such data units carry a variety of names in the context of different communication systems and communication protocols, such as packets, frames, segment, protocol data units, etc. The term “data unit” as used in the present specification and claims, generically refers to any such division of a data amount.
In order to ensure the complete and correct transmission of data units from a transmitting to a receiving protocol peer, a mechanism referred to as HARQ (Hybrid Automatic Repeat reQuest) is often used. HARQ mechanisms are commonly part of link layer protocols such as the Radio Link Control (RLC) protocol or the Medium Access Control (MAC) protocol specified for the Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) as well as for the Evolved-UTRAN. When using an HARQ mechanism, the receiver of data units sends reports, i.e. feedback reports, to the sender, such that the sender can determine whether sent data units were properly received, and if not, to appropriately perform retransmissions of data units.
A feedback report is a control data unit that is typically sent from the receiving entity of an HARQ protocol to the transmitting peer entity.
Feedback report s are often referred to as status message, status report, status, etc. They may have different formats depending on the protocol specification. Known implementations of such feedback reports comprise one or more references to protocol data units, or parts thereof, received or expected by the receiving protocol entity. These references are typically denoted as positive and/or negative acknowledgements and also referred to as ACK or NACK. An acknowledgement provides the transmitting protocol entity with information about successful or unsuccessful reception of one or more data units at the receiving protocol entity. Many of the known HARQ protocols assign a so-called sequence number (SN) to each data unit and use this sequence number as reference in status messages. A positive acknowledgement for the data unit with a given sequence number may then be referred to as ACK whereas a negative acknowledgement may be denoted as NACK. Widely, known protocols use lists and/or bitmaps in feedback reports. An acknowledgement may be explicit, i.e., represent the state of one particular data unit or it may be cumulative, i.e., provide information about the state of a collection of data units.
Time Division Duplexing (TDD) is a transmission scheme that allows an asymmetric flow for uplink and downlink transmission which is well suited to data transmission. In a TDD system, a common carrier is shared between the uplink and downlink, the resource being switched in time. Users are allocated one or more timeslots for uplink and downlink transmission. In TDD system, the Down Link/Up Link (DL)/(UL) asymmetry scenario makes the ACK/NACK design for HARQ a hard problem. In the scenarios of heavy DL transmission or heavy UL transmission, multiple ACK/NACKs per user within a TDD frame have to be treated in the limited UL or DL control channel respectively. One way to reduce the number of ACK/NACK reports is to compress the multiple ACK/NACK reports, e.g. by bundling method. In current LTE rel-8, multiple ACK/NACK can be compressed by ‘AND’ operation for each code-word across multiple subframes, namely bundling method.
However, after the compression of multiple ACK/NACK reports, it is hard to distinguish which ACK/NACK feedback corresponds to which Transmission Time Interval (TTI) and Discontinuous Transmission (DTX) cannot be reported in a good way. DTX means that a missing DL assignment occurs. E.g. eNodeB schedules a subframe indicated by downlink assignment in physical control channel (PDCCH) for a certain user equipment but the user equipment misses the downlink assignment in PDCCH. In ACK/NACK bundling mode, eNodeB is hard to know whether the user equipment missed some downlink assignments across multiple subframes or not. DTX report is a way for eNodeB to know whether the user equipment missed downlink assignments or not.
It has been found that the misdetection of DTX will cause a serious throughput loss above Transport Control Protocol (TCP) layer, due to the burst large delay caused by Radio Link Control (RLC) layer ARQ. (HARQ is in MAC layer, and ARQ is in RLC, which means higher layer RLC will also check the correctness of downlink data transmission.) RLC is a link-layer protocol that is responsible for error recovery and flow control in 3G (UMTS) cellular systems. 1% detection error of DTX to ACK will cause around 20% TCP throughput loss.